The goal of this project is to develop a prototype microwave phased array applicator for local tumor hyperthermia therapy. The therapeutic effectiveness of ionizing radiation therapy can be improved by the focused application of heat to the tumor. The use of microwaves for the generation of local hyperthermia has been successfully applied in both animal and clinical studies. However, current designs for microwave applicators heat only superficial tumors and must be mechanically moved in order to change the power distribution in the tumor and surrounding normal tissues. The specific aims of the proposed Phase I study are designed to develop new techniques for producing local tumor hyperthermia using microwaves. Preliminary studies have resulted in a new design for a microwave phased array hyperthermia applicator. The proposed Phase I studies will allow development of this existing design into a working prototype microwave hyperthermia applicator system. This applicator system will consist of a microwave phased array applicator, a microwave power amplifier and distribution network, and a computer control system for the applicator. This new microwave phased array applicator system will have the capability of focusing microwave energy more effectively in tissue than presently available clinical devices and the location of the focus will be electronically rather than mechanically scanned in order to produce a uniform tumor temperature distribution without overheating surrounding normal tissues. The results of the Phase I studies will provide a basis for human clinical trials of combined radiation and hyperthermia therapy.